The present invention relates to optical communications and more particularly to a method for optically synchronizing an optical transmitter and receiver in a communication system having multiple users.
Optical communication systems have been developed that provide fast and reliable means for passing information from a transmitter to a receiver. Adaptations to systems to provide access by multiple users include frequency division (FDMA) and time division (TDMA) multiplexing techniques. A further adaptation for multiplexing in an optical communications system is an analog of code division multiple access (CDMA) techniques used in radio frequency systems; in an optical system a light pattern having a spatial spread of dark and light patterns can be used. Such a spatial pattern of dark and light is commonly referred to as a speckle pattern, or alternatively as a mask. An optical communications system providing for transmission of such speckle patterns or masks is described in application Ser. No. 7/932,819, which is assigned to the assignee herein and incorporated herein by reference. In such a spatial CDMA type system multiple users can access the system simultaneously with optical signals having common frequencies; the respective signals are identified by distinctive spatial light patterns detected by the receiver.
A spatial CDMA type system has the potential for simultaneously handling the respective signals of numerous users provided that the system can accommodate complex speckle patterns. One important aspect of operation of such a system is spatial synchronization between the optical receiver and the optical transmitter. It is desirable that the optical communication system be able to rapidly and accurately detect and spatiaily align signals from respective users transmitting on the system.